Keeping windows and other glass surfaces clean is a relatively time-consuming and laborious undertaking. In particular, keeping a large number of windows clean is burdensome and, in many cases, is quite expensive. For example, it takes a great deal of time and expense for window washers to regularly clean all the windows of modern glass towers. Even in private homes, the effort involved in keeping windows clean is substantial.
A great deal of work has been done with the goal of developing self-cleaning coatings for windows and other substrates. One major area of research has focused on photocatalytic coatings. Research in this area is founded on the ability of photocatalytic coatings to break down organic materials that come into contact with the coatings. The most powerful photocatalyst appears to be titanium oxide (e.g., titanium dioxide). However, other semiconductors are also known to exhibit photocatalytic properties.
Generally, photocatalytic reactions occur when a semiconductor is irradiated with radiation having a higher energy than the band gap energy of the semiconductor. When radiation that strikes the surface of a photocatalytic coating comprising a semiconductor reaches or exceeds the band gap energy of the semiconductor, an electron is lifted from the valence band to the conduction band, creating a corresponding hole. Some of the excited electrons in the conduction band and some of the holes in the valence band recombine and dissipate the input energy as heat. The lifetime of other holes, however, is sufficient for them to travel to the surface of the coating, where they react with molecules absorbed on the surface of the coating, forming radicals (e.g., OH— and O2—) that can decompose organic matter at the surface of the coating.
Windows would derive great benefit from photocatalytic coatings. For example, such windows would have self-cleaning characteristics. To the extent organic matter is deposited on such a window, the photoactive coating would oxidize the organic deposits, thereby having a cleaning effect on the surface of the window. To the extent residue survives this photocatalysis, such residue would be more easily removed by washing or, for outdoor window surfaces, by run-off rainwater. Thus, a self-cleaning window would be highly desirable.
Photocatalytic coatings typically comprise one or more layers of thin film. This film is desirably selected to impart a number of different properties in the coating. For example, a titanium oxide film is commonly provided to impart photoactivity in the coating. In addition, one or more transparent dielectric films underlying the titanium oxide film are commonly provided. This underlying film desirably serves a number of functions and imparts various advantageous properties in the coating.
For example, the underlying film desirably serves as a good diffusion barrier. Glass substrates typically contain alkali metal ions that can migrate out of the glass and into the photocatalytic film. With soda-lime glass, for example, sodium ions can migrate out of the glass. Film beneath the photocatalytic layer can be used to provide a diffusion barrier that effectively seals the glass and prevents sodium ions from migrating into the photocatalytic layer. Sodium ions can adversely impact the self-cleaning properties of a photocatalytic film. Therefore, the photocatalytic film in a self-cleaning coating is preferably provided with at least some underlying film that is amorphous, dense, or otherwise effective as a diffusion barrier. This is particularly desirable for glass that must endure long term exposure to ultraviolet and/or is exposed to heat, such as during tempering or other heat treatment.
Further, the underlying film desirably has an inner interface that provides strong adhesion to the substrate. It is desirable to assure that the underlying film adheres well to the substrate, as this film serves as the foundation for the rest of the coating. It is also advantageous to minimize the stress in the photocatalytic coating as much as possible, since stress tends to subtract from the adhesive force of the coating. With photocatalytic coatings in particular, it is desirable to minimize stress and establish strong adhesion. Photocatalytic coatings tend to be #1 surface coatings. Therefore, they are typically exposed to more severe environmental conditions (e.g., conditions associated with an outdoor environment, such as periodic contact with rain) than other coatings, such as silver-based low-emissivity coatings, which are typically exposed to the protected atmosphere between the panes of an insulating glass unit. Thus, it is highly desirable to find ways to maximize the durability of photocatalytic coatings, so that delamination does not become a problem for these coatings.
In addition, the film beneath the photocatalytic layer desirably antireflects this layer as much as possible. A tradeoff is sometimes made in photocatalytic coatings whereby the film provided to achieve photoactivity has the effect of increasing the visible reflectance to a higher level than is ideal. As a consequence, windows bearing these coatings commonly have a somewhat mirror-like appearance. Unfortunately, this exaggerates the appearance of dirt on such windows. Thus, it is highly desirable to find ways to minimize the visible reflectance of photocatalytic coatings, so that these coatings do not show an exaggerated dirty appearance.
The film underlying the photocatalytic layer desirably also promotes good color properties in the coating. To the extent a photocatalytic coating has a colored appearance, it is pleasing if the coating exhibits a hue that is blue or blue-green. In most cases, it is preferable to provide a photocatalytic coating that is as color neutral (i.e., colorless) as possible. Thus, the film underlying the photocatalytic film can be used to modify the appearance of the coating (e.g., to modify its color and/or visible reflection).
Unfortunately, it is difficult to optimize all the foregoing coating properties using a single underlying film of any one material. As an alternative, the underlying film can be formed of two or more discrete layers of different materials, each chosen to optimize one or more of the desired coating properties. This, however, leaves the underlying film with at least one additional interface, which, as described below, is preferably avoided.
Many photocatalytic coatings are multi-layer coatings each comprising a plurality of discrete film layers. Each discrete layer typically is homogenous. That is, each layer typically has a composition that is uniform across the thickness of the layer. While discrete, homogenous layers have gained widespread acceptance, they have significant limitations. For example, the stress and adhesion properties are limited for a photocatalytic coating comprising a plurality of discrete film layers. This is due in part to the discrete interfaces that exist between discrete film layers. Unfortunately, stress tends to pile up at each discrete interface in a coating. Therefore, each discrete interface is a potential delamination site that is preferably avoided. Further, the optical opportunities are limited for photocatalytic coatings that comprise discrete, homogenous film layers. A coating of this nature can only achieve limited antireflection and color properties due to the optical limitations of providing each film in the coating as a discrete, homogenous layer.
It would be desirable to provide a photocatalytic coating in which the foregoing limitations are avoided while the desired coating properties are achieved.